I'm assuming that you do not mean commercial fishing but recreational/survival fishing.
Assuming that these are similar to real-world trout, you could look at how trout has been fished throughout history in the real world.
Stone-age peoples or survival fishing in shallow waters could include:
(1) using net-like construtions (weirs) made from tied/woven reeds, stretched across an area of a stream, high enough to stick above the water, anchored to the sand using poles (you can see a video of what this looks like here:
http://www.intothewildwest.com/native-americans-traditional-fishing-techniques/)
(2) spear fishing (either from shore, standing in wather, or by canoe; also spear fishing at night with torches)
(3) bone or wooden hooks and string using whatever bait known to be effective
With medieval levels of technology you would have more effective barbed metal hooks and you would see angling not disimilar to fishing today with a cane pole. Lines would likely be made of nettle. Hooks would be iron. Sinkers would be small bits of flat stone with a hole drilled through them for the line to pass through. A "commercial" version at the ocean would be "long lining", where you have a long line stretched across a river between to poles with a number shorter lines hanging down with hooks set up near the shore. When the tide goes out, you would go and collect the hooked fish. A similar method could be done by boat using floats on one end and the other end tied to the book and then you would just pull in the line every now and then to get the fish (
link to an illustration of long lining).
In medieval times you would also have more sophisticated fish nets and fish traps (see proceeding link for pictures). Spear fishing was also practices in medieval europe, using barbed metal spear tips and tridents.
Fly fishing is another possibility and a method that adventurer could use. Fly fishing is recorded as far back as roman times (
https://en.wikipedia.org/wiki/Fly_fishing)
For fantasy worlds some other options spring to mind:
* trained bears, bear companions, or druids wild shaped into bears could slap out fish from streams at especially shallow areas - OR - instead of bears, otters. See video:
https://www.youtube.com/watch?v=aaGrOqDKhdE
* dancing lights would be very helpful with night fishing and could perhaps grant a bonus or advantage or be treated as a helping action. Faerie Fire would be similarly useful. Other spells useful to help with fishing: charm animal, locate animals or plants, call lightning, and other lightning spells)
